AMLCD panels are typically utilized as screens of monitors, typically for laptop and smaller personal computers, as pocket televisions and as displays of other types of systems, such as for avionic displays. Future applications may include home televisions.
A portion of a typical AMLCD panel is illustrated in FIGS. 1A-1D. The AMLCD panel is a two-dimensional array of switches 10, each connected to an LCD pixel, all fabricated onto a base plate. The switches are typically thin film transistors (TFTs) made of polysilicon, amorphous silicon or cadmium selenide. Each switch 10 has gate and source electrodes 11 and 13, which are accessed by gate and source signal busses 12 and 14, respectively. The gate and source busses 12 and 14 are activated by scan and data drivers, respectively, and are arranged in a row and column geometry as shown in FIGS. 1A and 1B. Each switch 10 also has a drain electrode 16 to which is connected an electrode 18 of a LCD pixel. The electrode 18 typically is mostly square, with a step 21 cut out in the upper left corner. Step 21 is typically just large enough to accommodate switch 10 and its associated elements.
FIGS. 1C and 1D illustrate an enlarged section of the switch 10, in top view and cross-section, respectively, for a gate-down TFT design. Typically, the switch 10 comprises an insulator 24 typically made of silicon nitrite and a semiconductor 26 formed of intrinsic amorphous silicon covered by an n type amorphous silicon 0.3 micrometers thick. The source and gate electrodes 11 and 13 are formed of aluminum and the drain electrode 16 is formed of indium-tin-oxide. Typically the switch 10 and its associated elements are deposited on a glass plate 28.
A finished AMLCD panel includes many layers placed over the active matrix shown in FIGS. 1A-1D. Typically, over the active matrix a liquid crystal layer is placed, over which are placed a grid layer, a filter layer, a polarizer layer and a cover layer. It is noted that the additional layers are all transparent to light.
A high definition color display may contain 1024 columns per color and about 700 rows, providing an AMLCD panel of 2,150,400 switches. Producing such an array of transistors on a glass plate 28 of about 20.times.15 square inches, without a defect, is formidably difficult. Some defects are to be expected and thus, a repair methodology is typically provided with the manufacturing process.
Designing for redundancy in order to repair microelectronic memory circuits was described as early as 1972 by S. E. Schuster in The IBM Technical Disclosure Bulletin, Vol. 15 (2), 1972, p. 571. A key feature of the design method described therein is that a small number of spare cells are manufactured at the bottom of the array, or at some other location outside of the main array locations. The number of spare cells should correspond to the expected number of defects in the array and is typically a fairly small percentage of the total number of cells in the array. When a defect in the main array is located, the defective rows are isolated and the spares are activated via laser cutting.
The design methodology described by S. E. Schuster cannot be applied to AMLCD panels because spare cells at a remote location cannot compensate for a visual defect existing somewhere within the main array. Thus, each spare cell must be at the location of the defective cell.
Repair methodologies have been proposed for AMLCD panels. A common denominator of these methods is that a defective cell is not usually repaired; rather, it is isolated so that it does not effect the overall appearance of the display. For example, if the source and drain electrodes of a given cell are shorted, a defective column is produced which appears as a bright or dark vertical line on the screen. Such a cell cannot be repaired. However, if the source electrode 13 of the defective cell is cut from the source bus 14, the defect can be isolated to a single cell and, because of the small size of a single cell of the array, the defect may not be apparent.
The methodology described hereinabove is acceptable for second-quality products. However, consumers expect high quality products to be free from defects. For some applications, such as for avionic displays, no defects are acceptable.
The defect can also be locally repaired. For example, a break in a metal line can be repaired by depositing metal locally over the break via laser beam or ion beam induced chemical vapor deposition. Repair of a short between two metal lines can be achieved by ablating the metal bridge with a laser. Such a system is described by J. Henley in Solid State Technology, Vol. 35, April 1992, pp. 65-68.
Local defect repair methodologies require identification of the exact, microscopic location of the defect. It is not sufficient to identify the location of the defective cell; rather, the location of the defect within the cell must be determined. While the cell size is typically 100.times.100 .mu.m.sup.2, the defect size may be 1.times.1 .mu.m.sup.2. Hence, the imaging resolution required for local defect repair is at least a hundred times more than that needed to detect a defective cell. There is also a corresponding increase in detection time and repair cost.